Previously, we relied on the plugin to close the DB connections
on shutdown. While this keeps the code simple, there is no guarantee
that the plugin author will remember to close the DB.
In that case, it's better to track the connections from the server side
and close them in case they weren't closed already. This complicates
the API slightly, but it's a price we need to pay.
https://mattermost.atlassian.net/browse/MM-56402
```release-note
We close any remaining unclosed DB RPC connections
after a plugin shuts down.
```
Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>
Co-authored-by: Mattermost Build <build@mattermost.com>
